Incremental position-measuring devices which have proven effective are those where the measuring standard has a first grating in the form of a periodic incremental graduation, and where the scanning unit has at least one second grating disposed at a defined scanning distance from the grating of the measuring standard. For purposes of position measurement, the two gratings are illuminated with light of a particular wavelength, and a first detector array obtains a position-dependent electrical scanning signal therefrom.
A position-measuring device of this type is described, for example, in DE 33 34 398 C1, where incremental position measurement is complemented by absolute position measurement. In addition to the first grating, the measuring standard also includes an absolute mark, from which a second electrical scanning signal is generated by a second detector array, the second electrical scanning signal uniquely defining an absolute position within a measurement range.
The scanning distance of the first and second gratings is selected so as to achieve as high a degree of modulation as possible, and thus a high signal amplitude. Optimum scanning distances for incremental position measurement lie in the so-called Talbot planes. In the Talbot planes, repeating self-images of the first grating are formed, so that a periodic intensity distribution is obtained in these Talbot planes. In the so-called “two-grating encoder” according to DE 33 34 398 C1, the images of the first grating, which is illuminated by a plane wavefront, are formed in Talbot planes at the following distances Ln from the first grating:Ln=n×T12/λ    n: an integer number 0, 1, 2, . . .    T1: grating constant of the first grating    λ: wavelength of the light
It is apparent that the locations of these Talbot planes, and thus the optimum scanning distances, depend, on the one hand, on the grating constant of the first grating and, on the other hand, on the wavelength of the light used.
In German Patent DE 33 34 398 C1, it was already realized that relatively large scanning distances are advantageous for incremental scanning, but disadvantageous for the scanning of the absolute mark. German Patent DE 33 34 398 C1 proposes to solve this problem by providing different scanning distances for the two types of scanning, namely by providing a larger scanning distance for incremental scanning than for the scanning of the absolute mark.
To be able to maintain the required scanning distances in this system, complex guides are proposed.